Filler-containing polyurethanes with a high water absorption capacity in the form of aqueous polyurethane gels (hydrogels) have been disclosed in German Offenlegungsschrift 3,151,925, and wherein from 20 to 80% by weight of abrasives such as aluminum oxide, cerium oxide, tungsten dioxide, boron carbide, silicon carbide and asbestos powder with the optional addition of graphite, micro-glass beads, mica or short staple fibers and optionally also fungicides, dyes or coloring pigments are added. The materials are converted into elastic abrasive gels based on polyurethane (urea) by the reaction of hydrophilic isocyanate prepolymers (based on polyoxyalkylene ethers containing .gtoreq.30% by weight of oxyethylene groups) with aqueous slurries of abrasive, which slurries may contain polyamine. The hydrophilic character of the gels is due to the use of hydrophilic (containing oxyethylene groups) polyether polyols as starting materials. The gels contain water in the form of polyurethanes swelled with water.
German Offenlegungsschrift 2,347,299 and 2,521,265 describe water-swelled polyurethane (urea) gels which may be either homogeneous or foamed with CO.sub.2 and which may contain up to 50 percent by volume of fillers such as silicates, various types of silica, aluminium oxides, tin oxide, antimony trioxide, titanium dioxide, graphite and graphitized carbon, retort carbon, carbon black, pulverulent cements, color pigments, fibers and cellulose powders in addition to surface active agents and nutrients. The water absorption in the gels is due mainly to the use of hydrophilic polyether polyols containing .gtoreq.40% by weight of oxyethylene sequences.
German Offenlegungsschrift 3,103,499 discloses substantially anhydrous polyurethane gel masses which contain polyols as dispersing agents and may contain active substances and possibly dyes, pigments, fibers, inorganic fillers, metal powders, active carbon, cellulose powder and silica preparations. Gels of this kind which contain polyol release at least a major proportion of the dispersed polyols when in aqueous suspension.
In the polyurethane gels known in the art, it is essential to synthesize hydrophilic polyurethanes based on polyethers with high ethylene oxide contents in order to obtain a sufficient water absorption capacity. This frequently gives rise to problems of (i) reactivity of the hydrophilic polyether polyols (which in most cases are highly reactive), (ii) of mechanical gel strength when highly hydrophilic polyether polyols are used, and (iii) high cost.
Numerous processes have already been described for impregnating foams and foam particles by steeping the foams in a reactive component such as polyisocyanates and subsequently reacting them with other reactants such as water, polyols, polyamines or diamine vapors. See, for example, the processes described in German Offenlegungsschriften 3,039,146 and 2,131,206; Japanese Patent 50-103,571; French Patents 1,587,855 and 1,574,789; and U.S. Pat. Nos. 2,955,056, 3,114,772 and 4,315,996.
Foams may also be subjected to a liquid which causes them to swell. They may then be exposed to polyurethane reactants, whereby it is possible to harden and rigidify the foam and substances may be incorporated in the swelled foam matrix, e.g. by the processes described in French Patents 1,341,717, 1,587,855 and 1,574,789 and German Auslegeschrift 1,911,645. Matrix foams of this kind have typical foam characteristics even though they may have a different rigidity or elasticity or different chemical or mechanical properties.
Numerous other patent specifications describe the bonding or pressing of foam particles (preferably waste particles of flexible polyurethane foam) with polyisocyanates, isocyanate prepolymers, polyols, polyamines, water or other reactants (optionally with the addition of cork, fibers, cellulose powder, flame retarding agents, pigments, metal powder or carbon black) to produce novel composite materials which may be provided with or welded to coverings, films or metal plates. Composite materials of this kind are used, for example, as insulating panels, linings, mattresses or molded articles. Suitable processes for obtaining such products are described, for example, in German Offenlegungsschriften 2,940,260, 3,213,610, 2,908,161, and 3,120,121; British Patents 1,337,413, 1.540,076; U.S. Pat. 4,254,177; and Japanese Patent 57/028,180.
The only process which has achieved any technical importance, however, is the production of composite block foam from size reduced polyurethane foam, 10 to 20% by weight of isocyanate compounds, up to about 10% by weight of fillers and small quantities of water. In this process, the filler consists mainly of color pigments used to impart a uniform color to the foam obtained from the various batches which may originally have differing colors. The water used in the preparation of the composite foam serves as a reactant to convert the polyisocyanate groups into polyurea groups with evolution of carbon dioxide. The quantity of water is calculated to correspond substantially to the stoichiometric requirement of the isocyanates but is at most used in only a relatively small excess since otherwise, the removal of moisture from the composite blocks (which are 40 to 60 cm in thickness) would give rise to difficulties.
In the field of (biological) waste water purification, many processes have already been proposed with the object of increasing the degradation effect so as to obtain purified water which is, as far as possible, free from harmful substances. The various processes which have been attempted include a process of oxidation of the noxious substances with increased oxygen supply to the activated sludge as well as special oxidation (processes such as treatment with ozone or hydrogen peroxide).
The catalytic oxidation of the contents of the waste water by means of air and with the addition of active charcoal followed by a precipitation has been recommended (see, e.g. German Patent 2,239,406; German Offenlegungsschrift 3,025,353; A. Bauer et al, Chemie Technik, Number 6, pages 3-9 (1982); K. Fischer et al, GWF-Wasser/Abwasser, Number 2, pages 58-64 (1981); R. E. Perrotti et al, Chemical Engineering Progress (CEP), Volume 69 (11), 63-64 (1973); G. Wysocki et al, ZC Chemie Technik, 3 (6), 205-208 (1974); and 3rd Report, "Adsorptive Abwasserreinigung" (October, 1975) of the "Ausschuss Wasser und Abwasser" (Water and Sewerage Board) at VCIeV.
The above-mentioned processes, however, were either found to be technically too complicated and expensive or the degradation effect was found to be insufficient. The numerous attempts to use active carbon for the purification of water have hitherto failed, in spite of the increased efficiency of decomposition, since the active carbon, even in a bound granulated form, was excessively size reduced by the very slight currents which inevitably occur in settling tanks from time to time, with the result that the carbon particles were discharged. Successful attempts to maintain a sufficiently large quantity of active carbon to be effective and to bind the active carbon sufficiently while maintaining the bioactivity in the settling tanks have not to date been successful.
German Offenlegungsschriften 3,032,882 (EP-A 46,900) and 3,032,869 (EP-A 46,901) describe the use of a macroporous substance having a low specific gravity (10 to 200 kg/m.sup.3) as carrier material for nitrifying bacteria for use in activated sludge purification. These macroporous materials may be, for example, typical polyurethane foams. A similar use of such foam particles in a process and an apparatus for anaerobic biological waste water purification has also been described, see e.g. GWF-Wasser/Abwasser, 124 (1983), Number 5, pp. 233-239. Foams of this kind, however, float to the surface in open activated sludge tanks and give rise to various problems. Foam in the form of lumps based (inter alia) on polyurethanes have also been proposed for use in various special processes as bulk filling bodies (German Patent 2,839,872 and German Auslegeschrift 2,550,818) or a trickling filter mass (Austrian Patent 248,354) for biological waste water purification. The use of relatively abrasion resistant polyurethane (urea) foams having an open cell structure and a urea/urethane ratio below 5 as carrier medium for microbiologically active cells in waste water purification processes has been described in U.S. Pat. No. 4,503,150 which also mentions numerous other publications relevant to the state of the art describing the use of foams in biological waste water purification.
The use of polyurethane foam pieces as a filtration medium has been described in European Patent 77,411. In this process, the foam, when laden with dirt, is regenerated from time to time by a special process of rinsing.
The combination of microorganisms with surface active solids for increasing the activity of the microorganisms in bioconversion processes is also known. Thus, for example, German Offenlegungsschriften 2,633,259 and 2,703,834 describe the adsorption of cells on, for example, aluminium oxide, bentonites and SiO.sub.2 and their subsequent embedding in polyacrylates. Furthermore, German Offenlegungsschrift 2,629,692 describes the incorporation of cells in photo-hardenable polyurethanes which contain photo-hardenable acrylate double bonds.
The embedding of viable cells in polyurethane hydrogels has also been disclosed; see, for example, Tanaka et al, European Journal of Applied Microbiology and Biotechnology, 7, (1979), page 371 et seq. Furthermore, a process for the preparation of hydrophilic biocatalysts in the form of gels or foams highly charged with enzymatically active substance by the inclusion of whole cells, cell fragments or enzymes is known. The biocatalysts are prepared by mixing an aqueous suspension of enzymatically active substance with hydrophilic polyisocyanates to form an enzymatically highly active, hydrophilic polyurethane network in the form of blocks or beads (see German Offenlegungsschrift 2,929,872). Further publications relative to this state of the art are mentioned in said Offenlegungsschrift, on page 7. The immobilization of microbial cells in polyurethane matrices (such as polyurethane foams or gels) has been described by J. Klein, and M. Klug in Biotechnology Letters, Vol. 3, No. 2, pages 65-90 (1981). Cationic, polymer-containing hydrophilic polyurethane gels incorporating viable cells have been described in German Offenlegungsschrift 3,312,578.
The preparation of polyurethanes containing enzymatically active substances is difficult, however, and has the disadvantage that the high reactivity of the isocyanate groups causes at least partial killing of the bacteria or cells or inactivation of enzymatically active material. The residual activities are as low as, for example, 7 to 48%. It is therefore not advantageous to incorporate living bacteria in the preparation of hydrophilic polyurethanes which are to be used for the purification of waste water. The quantity of bacteria which can be incorporated in this way is limited and a high proportion of the bacteria is inactivated by isocyanate reactions. Furthermore, the continuous preparation of active polyurethane masses containing bacteria and the storage of such polyurethane masses in a viable condition involves problems of manufacture and storage if the settling tanks (which generally have a capacity of several thousand cubic meters) are to be supplied with the required quantity and concentration of bacteria incorporated in the polymers. The capacity for growth of the bacteria would be drastically reduced even if the bacteria were incorporated in situ in the purification plant since they can only survive for a short time under the conditions of immobilization in the reaction medium.
The problem therefore remained to be solved of developing suitable methods for the preparation of carrier materials for use in new, economical and efficient processes for improved waste water purification.
It is therefore an object of the present invention to provide polyurethane (urea) compositions which have a high filler content and high water absorption capacity, which do not float to the surface in water and which would be suitable for use as carriers for biomasses in biological waste water purification.